Britain's Queen Elizabeth II poses with her five great-grandchildren and her two youn|c2016 Annie Leibovitz/Handout via Reuters

Queen Elizabeth greeted thousands of well-wishers from across Britain and beyond as she celebrated her 90th birthday on Thursday, demonstrating the world's oldest monarch's intent to keep doing the job she has performed for more than six decades.

Usually the queen's birthday passes with little ceremony but to mark Thursday's milestone a beaming Elizabeth, attired in a light green outfit, mingled with crowds during a lengthy walkabout near her Windsor Castle home, west of London.

In a rare move in recent years, she then drove through Windsor in an open-topped car with Prince Philip, her husband of 68-years by her side.

"All of us are here to respect the queen and to show her our affection and how much we appreciate all her years of service and to wish her a happy birthday," said Donna Werner who had travelled to Windsor from Connecticut in the United States.

Werner, like many others in the crowd festooned with red, white and blue, had been camped out since before dawn along with three English friends she met during the 2011 wedding of Elizabeth's grandson William and his wife Kate.

Later the queen will light a beacon, the first of about 1,000 across Britain and worldwide to mark the occasion. There were also artillery gun salutes in London and other British cities, while the Houses of Parliament were to be illuminated later in red, white and blue.

"I send my best wishes to those who are celebrating their 90th birthday ... on this shared occasion, I send my warm congratulations to you," said a post on the queen's Twitter site.

Close aides say Elizabeth, who has been on the throne for 64 years and is by far the oldest monarch in British history, was far more interested in events to mark her 90th birthday than she had been about overtaking her great-great-grandmother Queen Victoria last September as Britain's longest-reigning sovereign.

Born on April 21, 1926, Elizabeth shows no signs of retiring let alone abdicating.

An Ipsos MORI poll last week found 70 percent wanted her to stay queen compared to 21 percent who thought she should abdicate or retire.

"HUMBLE ADDRESS"

"As the sands of culture shift and the tides of politics ebb and flow, Her Majesty has been steadfast, a rock of strength for our nation, for our Commonwealth, and on so many occasions, for the whole world," Prime Minister David Cameron said in a "humble address" in parliament.

Jeremy Corbyn, the ardent republican leader of the opposition Labour Party, paid tribute to her "outstanding commitment to public life".

"Whatever differing views people across this country have about the institution, the vast majority share an opinion that Her Majesty has served this country ... with a clear sense of public service and public duty," he said.

Some, though, were less overawed by the occasion.

"Headline polling figures mask what's really happening," said Graham Smith, chief executive of campaign group Republic. "With the republican movement stronger than ever - and growing - and with the succession looming on the horizon, the monarchy is in a more perilous position than pundits will tell you this week."